In many applications, thin layers are deposited on a substrate, e.g. on a glass substrate. The substrate is typically coated in a vacuum chamber of a coating apparatus. For some applications, the substrate is coated in a vacuum chamber using a vapor deposition technique. Over the last few years, the price of electronic devices and particularly opto-electronic devices has reduced significantly. Further, the pixel density in displays has increased. For TFT displays, a high density TFT integration is beneficial. In spite of the increased number of thin-film transistors (TFT) within a device, the yield is to be increased and the manufacturing costs are to be reduced further.
One or more structures or layers may be deposited on a substrate such as a glass substrate to form an array of electronic or optoelectronic devices such as TFTs on the substrate. A substrate with electronic or optoelectronic structures formed thereon is also referred to as a “sample” herein. During the manufacturing of TFT-displays and other samples, it may be beneficial to inspect one or more structures deposited on the sample to monitor the quality of the sample.
The inspection of the sample can, for example, be carried out by an optical system. However, the dimension of some of the features of the sample or the size of defects to be identified may be below the optical resolution, making some of the defects non-resolvable to the optical system. Charged particles such as electrons may be utilized for inspecting the surface of the sample which may provide a better resolution as compared to optical systems.
However, the inspection of samples with non-planar surfaces with a charged particle beam may be challenging, because not the whole sample surface may be located at the same distance from the objective lens, and the depth of field of a charged particle beam device is limited. Refocusing the charged particle beam modifies the working distance of the device and may introduce beam aberrations and/or measurement errors.
Accordingly, given the increasing demand for an increased quality of displays on large area substrates, there is a need for an improved method for investigating samples with a high measurement accuracy, with reduced beam aberrations, and in a quick and reliable manner. In particular, there is a need for inspecting non-planar large-area samples with a high measurement accuracy, e.g. when conducting critical dimension measurements.